How do I subtract from an RBG?

I am planning to make a Frame that has it's background color change via a button, but I can't figure out how to do math on RBG values.

Frame = script.Parent
Blue = Frame.BackgroundColor3.b

  

Frame.BackgroundColor3 = Color3.fromRGB(Frame.BackgroundColor3.r, Frame.BackgroundColor3.g, Frame.BackgroundColor3.b - 50)